mg4/server/mvt.go

package main

import (
	"errors"
	"math"
)

// The roads of a Mapbox Vector Tile (spec 2.1) in the OpenMapTiles schema: the line features
// of the "transportation" layer whose "class" the car draws (roadClasses), scaled to
// mapExtent. A minimal protobuf reader, enough for that and nothing more; anything malformed
// is an error, never a guess.

type roadLine struct {
	class  int
	points [][2]int
}

var errBadTile = errors.New("vector tile is malformed")

// pbReader walks protobuf fields.
type pbReader struct {
	b []byte
	i int
}

func (r *pbReader) done() bool { return r.i >= len(r.b) }

func (r *pbReader) varint() (uint64, error) {
	var v uint64
	for shift := uint(0); shift < 64; shift += 7 {
		if r.i >= len(r.b) {
			return 0, errBadTile
		}
		c := r.b[r.i]
		r.i++
		v |= uint64(c&0x7f) << shift
		if c < 0x80 {
			return v, nil
		}
	}
	return 0, errBadTile
}

// field reads one field's key and, for length-delimited ones, its bytes.
func (r *pbReader) field() (num int, wire int, val uint64, data []byte, err error) {
	key, err := r.varint()
	if err != nil {
		return 0, 0, 0, nil, err
	}
	num, wire = int(key>>3), int(key&7)
	switch wire {
	case 0:
		val, err = r.varint()
	case 1:
		if r.i+8 > len(r.b) {
			return 0, 0, 0, nil, errBadTile
		}
		r.i += 8
	case 2:
		var n uint64
		n, err = r.varint()
		if err == nil {
			if n > uint64(len(r.b)-r.i) {
				return 0, 0, 0, nil, errBadTile
			}
			data = r.b[r.i : r.i+int(n)]
			r.i += int(n)
		}
	case 5:
		if r.i+4 > len(r.b) {
			return 0, 0, 0, nil, errBadTile
		}
		r.i += 4
	default:
		return 0, 0, 0, nil, errBadTile
	}
	return num, wire, val, data, err
}

// packed reads a repeated uint32 field, packed or not.
func packed(wire int, val uint64, data []byte, into []uint32) ([]uint32, error) {
	if wire == 0 {
		return append(into, uint32(val)), nil
	}
	if wire != 2 {
		return nil, errBadTile
	}
	r := pbReader{b: data}
	for !r.done() {
		v, err := r.varint()
		if err != nil {
			return nil, err
		}
		into = append(into, uint32(v))
	}
	return into, nil
}

// decodeRoads returns the drawable roads of the tile.
func decodeRoads(tile []byte) ([]roadLine, error) {
	r := pbReader{b: tile}
	for !r.done() {
		num, _, _, data, err := r.field()
		if err != nil {
			return nil, err
		}
		if num != 3 { // layers
			continue
		}
		lines, isRoads, err := decodeLayer(data)
		if err != nil {
			return nil, err
		}
		if isRoads {
			return lines, nil
		}
	}
	return nil, nil // a tile without roads (sea, desert) is an empty one
}

type rawFeature struct {
	tags     []uint32
	geomType uint64
	geometry []uint32
}

func decodeLayer(layer []byte) ([]roadLine, bool, error) {
	r := pbReader{b: layer}
	var name string
	var keys []string
	var values []string // only string values matter for "class"
	var features []rawFeature
	extent := uint64(4096)
	for !r.done() {
		num, wire, val, data, err := r.field()
		if err != nil {
			return nil, false, err
		}
		switch num {
		case 1:
			name = string(data)
		case 2:
			f, err := decodeFeature(data)
			if err != nil {
				return nil, false, err
			}
			features = append(features, f)
		case 3:
			keys = append(keys, string(data))
		case 4:
			values = append(values, valueString(data))
		case 5:
			if wire != 0 || val == 0 {
				return nil, false, errBadTile
			}
			extent = val
		}
	}
	if name != "transportation" {
		return nil, false, nil
	}
	scale := float64(mapExtent) / float64(extent)
	var lines []roadLine
	for _, f := range features {
		if f.geomType != 2 { // LINESTRING
			continue
		}
		class, ok := -1, false
		for i := 0; i+1 < len(f.tags); i += 2 {
			k, v := int(f.tags[i]), int(f.tags[i+1])
			if k >= len(keys) || v >= len(values) {
				return nil, false, errBadTile
			}
			if keys[k] == "class" {
				class, ok = roadClasses[values[v]]
				if !ok {
					class = -1
				}
			}
		}
		if class < 0 {
			continue
		}
		parts, err := lineGeometry(f.geometry, scale)
		if err != nil {
			return nil, false, err
		}
		for _, p := range parts {
			lines = append(lines, roadLine{class: class, points: p})
		}
	}
	return lines, true, nil
}

func decodeFeature(data []byte) (rawFeature, error) {
	var f rawFeature
	r := pbReader{b: data}
	for !r.done() {
		num, wire, val, d, err := r.field()
		if err != nil {
			return f, err
		}
		switch num {
		case 2:
			if f.tags, err = packed(wire, val, d, f.tags); err != nil {
				return f, err
			}
		case 3:
			f.geomType = val
		case 4:
			if f.geometry, err = packed(wire, val, d, f.geometry); err != nil {
				return f, err
			}
		}
	}
	if len(f.tags)%2 != 0 {
		return f, errBadTile
	}
	return f, nil
}

// valueString is a Value's string_value, "" for any other kind.
func valueString(data []byte) string {
	r := pbReader{b: data}
	for !r.done() {
		num, _, _, d, err := r.field()
		if err != nil {
			return ""
		}
		if num == 1 {
			return string(d)
		}
	}
	return ""
}

// lineGeometry follows the MoveTo/LineTo commands of a (multi)line string.
func lineGeometry(cmds []uint32, scale float64) ([][][2]int, error) {
	var parts [][][2]int
	var cur [][2]int
	x, y := 0, 0
	zig := func(v uint32) int { return int(int32(v>>1) ^ -int32(v&1)) }
	at := func() [2]int {
		return [2]int{int(math.Round(float64(x) * scale)), int(math.Round(float64(y) * scale))}
	}
	for i := 0; i < len(cmds); {
		id, count := cmds[i]&7, int(cmds[i]>>3)
		i++
		switch id {
		case 1, 2: // MoveTo, LineTo
			if i+2*count > len(cmds) {
				return nil, errBadTile
			}
			for k := 0; k < count; k++ {
				x += zig(cmds[i])
				y += zig(cmds[i+1])
				i += 2
				if id == 1 {
					if len(cur) >= 2 {
						parts = append(parts, cur)
					}
					cur = [][2]int{at()}
				} else {
					if p := at(); len(cur) == 0 || cur[len(cur)-1] != p {
						cur = append(cur, p)
					}
				}
			}
		case 7: // ClosePath, not in a line; ignored
		default:
			return nil, errBadTile
		}
	}
	if len(cur) >= 2 {
		parts = append(parts, cur)
	}
	return parts, nil
}